Radiation detectors using simple planar electrodes and which are based on ionization measurements often suffer from poor collection of charge carriers of certain polarity types. Such detectors include, for example, semiconductor detectors, liquid ionization detectors, and gas ionization detectors. The poor collection characteristics of these detectors can be due to such factors as intrinsic material properties, defects in the detector medium, or radiation damage. For example, in certain semiconductor detectors, positive charge carriers, holes, migrate through the detector medium at a much slower rate than the negative charge carriers, electrons. Additionally, in certain semiconductor detectors, holes are more likely to become trapped within the detector medium. As a result, such detectors produce signals that vary in amplitude depending upon the location within the detector at which incident radiation interacts with the detector medium.
Specifically, if incident radiation is absorbed very close to the cathode of a detector, generated holes need only travel a short distance before being collected at the cathode. Corresponding generated electrons must travel a much greater distance through the detector medium before being collected at the anode. In such an example, the rapid migration rate and good collection efficiency of the electrons allows the detector to produce a full amplitude signal. If, on the other hand, the incident radiation is absorbed very close to the anode of the detector, generated holes must travel through almost the entire length of the detector medium before being collected at the cathode. Corresponding generated electrons only travel a short distance before being collected at the anode. Due to the poor migration characteristics of the holes, a weak signal is generated thus resulting in a reduced signal amplitude. Variation in signal amplitude results in poor energy resolution.
In one attempt to overcome such position dependent signal amplitude variation problems, Frisch grids have been implemented in liquid and gas ionization detectors. Frisch grids provide for the sensing of only charge carriers of a single selected polarity. In so doing, problems such as poor migration characteristics of certain polarity charge carriers can be negated. A Frisch grid consists of a mesh-like electrode positioned within the liquid or gaseous detector medium in close proximity to the collecting electrode. A voltage potential opposite that of the selected charge carrier is applied to the Frisch grid. The magnitude of the voltage potential applied to the Frisch grid is less than the voltage potential applied to the collecting electrode. Signals are derived from carriers which pass between the grid and the adjacent end electrode. In so doing, such signals are not dependent upon the location at which the single polarity charge carriers are generated within the main detector volume. Additionally, when using a Frisch grid, the signal amplitude will depend only upon the collection of a charge carrier of a single polarity type.
However, Frisch grids are not without drawbacks. The use of Frisch grids may result in imperfect charge carrier transmission. That is, some of the charge carriers may not pass through the Frisch grid as desired but, instead, are "trapped" at the surface of the Frisch grid. Such trapped carriers lead to loss of signal strength and degraded resolution. Additionally, Frisch grids are not well suited for use in semiconductor ionization detectors.
In another attempt to alleviate position dependent signal amplitude variation problems, hemispherical electrodes have been used in semiconductor detectors. Although such electrodes have been found to achieve a certain degree of preferential single polarity charge carrier sensing, such a configuration renders detector fabrication extremely difficult. Furthermore, the use of hemispherical electrodes often results in the creation of a highly non-uniform electric field within the detector. In turn, the highly non-uniform electric field often prevents good charge collection.
Consequently, a need exists for an ionization detector suitable for single polarity charge carrier sensing which does not trap charge carriers, does not produce a highly non-uniform electric field within the ionization detector medium, does not dramatically increase ionization detector fabrication costs, which is suitable for use in semiconductor ionization detectors, and which does not suffer from position dependent signal amplitude variation problems.